H-NOX proteins (named for Heme-Nitric oxide and OXygen binding domain) are members of a highly-conserved, well-characterized family of hemoproteins (Iyer, L M et al. (2003) BMC Genomics 4(1):5; Karow, D S et al. (2004) Biochemistry 43(31): 10203-10211; Boon, E M et al. (2005) Nature Chem. Biol. 1:53-59; Boon, E M et al. (2005) Curr. Opin. Chem. Biol. 9(5):441-446; Boon, E M et al. (2005) J. Inorg. Biochem. 99(4):892-902; Cary, S P et al. (2005) Proc Natl Acad Sci USA 102(37): 13064-9; Karow D S et al. (2005) Biochemistry 44(49):16266-74; Cary, S P et al. (2006) Trends Biochem Sci 31(4):231-9; Boon, E M et al. (2006) J Biol Chem 281(31):21892-902; Winger, J A et al. (2007) J Biol Chem. 282(2):897-907). H-NOX proteins are nitric-oxide-neutral, unlike previous hemoglobin-based oxygen carriers, H-NOX do not scavenge circulating nitric oxide, and thus are not associated with hypertensive or renal side effects. The intrinsic low NO reactivity (and high NO stability) makes wild-type and mutant H-NOX proteins desirable blood substitutes because of the lower probability of inactivation of H-NOX proteins by endogenous NO and the lower probability of scavenging of endogenous NO by H-NOX proteins. Importantly, the presence of a distal pocket tyrosine in some H-NOX proteins (Pellicena, P. et al. (2004) Proc Natl. Acad Sci USA 101(35): 12854-12859) is suggestive of undesirable, high NO reactivity, contraindicating use as a blood substitute. For example, by analogy, a Mycobacterium tuberculosis hemoglobin protein, with a structurally analogous distal pocket tyrosine, reacts extremely rapidly with NO, and is used by the Mycobacterium to effectively scavenge and avoid defensive NO produced by an infected host (Ouellet, H. et al. (2002) Proc. Natl. Acad. Sci. USA 99(9):5902-5907). However, it was surprisingly discovered that H-NOX proteins actually have a much lower NO reactivity than that of hemoglobin making their use as blood substitutes possible.
It was discovered that H-NOX proteins that bind NO but not O2 can be converted to H-NOX proteins that bind both NO and O2 by the introduction of a single amino acid mutation (see WO 2007/139791 and WO 2007/139767). Thus, the affinity of H-NOX proteins for O2 and NO and the ability of H-NOX proteins to discriminate between O2 and NO ligands can be altered by the introduction of one or more amino acid mutations, allowing H-NOX proteins to be tailored to bind O2 or NO with desired affinities. Additional mutations can be introduced to further alter the affinity for O2 and/or NO. The H-NOX protein family can therefore be manipulated to exhibit improved or optimal kinetic and thermodynamic properties for O2 delivery. For example, mutant H-NOX proteins have been generated with altered dissociation constants and/or off rates for O2 binding that improve the usefulness of H-NOX proteins for a variety of clinical and industrial applications. The ability to tune H-NOX proteins to bind and deliver O2 is a therapeutic avenue that addresses and overcomes the central shortcomings of current O2 carriers.
H-NOX proteins are relatively small in size and may be filtered through the kidneys resulting in a short circulation half-life. What is needed for certain therapeutic uses is an H-NOX with a longer circulation half-life that can bind and deliver O2 and/or NO to distal tissues for sufficient periods of time. Provided herein are polymeric H-NOX proteins with a longer circulation half-life. Additionally, H-NOX proteins extravasate into tumors where they accumulate at different rates. Polymeric H-NOX proteins are tuned to transport oxygen through normoxic regions of tumors and release oxygen deep within hypoxic zones within tumors. This combination of features represents a significant advance in the use of oxygen carriers as modifiers of the hypoxic niches of tumors to increase the efficacy of radiotherapy, chemotherapy and other anti-cancer treatments reliant on oxygenation of tumor cells.
All references cited herein, including patent applications and publications, are incorporated herein by reference in their entirety.